1. Field of the Invention
This invention relates to a zone melting method by means of laser, and particularly to a zone melting method wherein high temperature-gradient is built up on a thin film or semiconductor thin film due to the irradiation of laser beam to carry out the zone melting, and the surface roughness of the thin film after the zone melting can be minimized. In addition, this invention also relates to a zone melting apparatus to be used in the aforementioned zone melting method.
2. Brief Description of the Prior Art
In the past, it was well known that the zone melting can be performed by locally melting the semiconductor thin film such as InSb on a substrate by use of laser beams. One example is reported in the literature "R. Traniello Gradassi, ALTA FREQUENZA, n.7 vol XLIV-1975, page 350-353".
According to the above-mentioned zone melting method, the grains of the crystallites are generally made coarse and the removing of impurities is also conducted, so that the thin film of InSb having a property like single crystals can be obtained. In this case, if infrared rays emitted by a CO.sub.2 laser and the like are used, the zone melting of the InSb thin film can be readily carried out even under a high temperature-gradient ranging from 400.degree. C./cm to 2000.degree. C./cm.
The zone melting under a high temperature-gradient dT/dx in the direction (x direction) along which the melting zone travels results in the effective refining of the crystallite by the segregation of native impurities and also in the elimination of voids and defects and others, so that highly-pure semiconductor thin films without crystal imperfection will easily be obtained.
In gaseous lasers using molecules such as CO.sub.2, CO, H.sub.2 O or HCN, which generally require a high voltage power source, there is the fluctuation of laser-output P in the range of 1-5% with the frequency of several hertz to several hundreds hertz. As the result, the temperature in the melting zone during the zone melting process will fluctuate in the range of about 5.degree.-25.degree. C. for 525.degree. C. (melting point of InSb crystal), and about 14.degree.-70.degree. C. for 1410.degree. C. (melting point of Si crystal). This phenomenon is enhanced by the fact that the heat capacity and the volume to be melted in the case of the thin film semiconductor is extremely small (e.g. 100 .mu.m.times.10mm.times.1mm) in contrast to the case of the zone-melting of bulk semiconductors. The small volume and the resultant small heat capacity, therefore, directly lead to the temperature-fluctuation of the melting zone.
The generation of crystal imperfection and non-stoichiometric excess atoms in compound semiconductors caused by the aforementioned fluctuation of temperature T and also by the undesirable variation of temperature-gradient dT/dx in the direction of zone melting (x-direction) is the first disadvantage in a conventional laser thin film zone melting apparatus.
Furthermore, the laser beam having the cross-section of 8 mm width and 5 mm length, that is concentrated on the thin film surface, will have higher energy-density at the central portion and lower energy-density at its periphery. Such inhomogeneous distribution of energy density within the beam cross-section results in the non-uniformity of temperature in the melting zone during the zone melting process. This directly leads to the undesirable temperature-gradient dT/dy in the direction (y direction) perpendicular to the direction of zone-melting (x direction). In such cases, it has been found that the thickness corrugation with surface roughness ranging 0.1-10 .mu.m is formed on the semiconductor thin film during and after the zone melting.
Accordingly, the second disadvantage in a conventional laser thin film zone melting apparatus lies in the fact that the formation of rough surfaces, i.e. wrinkles of more than 0.1 .mu.m, can not be avoided. The formation of such wrinkles on the surface of thin films in the course of zone-melting results in the local inhomogeneity of the temperature due to inhomogeneous heat conduction (in case of heating and cooling). This phenomenon causes non-stoichiometric excess atoms of the costituents, dislocation, crystal (point) defect, and voids within the thin film crystal.